1. Field of the Invention
This invention relates to semiconductor manufacturing technology generally, and more specifically, to heat spreader technology for heat dissipation in a semiconductor package.
2. Description of the Related Art
There is a trend toward increasing the number of functions built into a given integrated circuit (also referred to as a device.) This results in an increased density of circuits in the device. Along with the increased circuit density, there is always a desire to increase the data processing speed; therefore, the clock speed of the device is increased as well. As the density of circuits and the clock speed increase, the amount of heat generated will increase. Unfortunately, device reliability and performance will decrease as the amount of heat that the device is exposed to increases. Therefore, it is critical that there be an efficient heat-removal system associated with integrated circuits.
FIG. 1 illustrates a typical integrated circuit and associated packaging. There are a number of methods for removing heat from integrated circuits 103, including active methods, such as fans or recirculated coolants (not shown), or passive methods, such as heat sinks 106 and heat spreaders 105. Because of decreasing device 103 size, there is usually a need to evenly distribute heat generated by the small device 103 to the larger heat sink 107 to eliminate “hot spots.” This is the function of heat spreaders 105. Heat spreaders 105 are coupled to the integrated circuit through the use of a thermally conductive material 104. These thermal interface materials 104, such as gel or grease containing metal particles to improve heat conduction, are applied in between the device 103 and the heat spreading structure 105 to improve the heat transfer from the integrated circuit 103 to the heat spreader 105. Typically, the heat spreading structure 105 will be constructed either of a ceramic material or a metal, such as aluminum or copper. Aluminum is preferred from a cost standpoint, as it is easy and cheap to manufacture; however, as the heat load that needs to be transferred increases, copper becomes the metal of choice because of its superior heat transfer characteristics. There will typically be a contiguous wall 106 around the edge of the heat spreader, which serves as a point of attachment and support between the substrate 101 and the spreader 105. The contiguous wall 106 completely surrounds the area containing the device, forming an enclosed cavity on top of the substrate 101. There is often a heat sink 107 attached to the heat spreader 105 using a thermal interface material 108, to allow for the greater cooling capacity associated with the high-surface area of the heat sink 107.
While heat spreaders have proven to be effective in increasing heat dissipation efficiency, there are problems with the current design that lead to decreased manufacturing yields, as well as higher packaging costs. FIGS. 2a–2b illustrate these problems.
First, the contiguous wall 106a associated with the heat spreader body 105a adds little to the dissipation of heat. Its main purpose is to act as a support for the heat spreader body 105a, and as a point of attachment to the substrate 101a. This adds excess weight and cost, as well as contributes to manufacturing defects through the addition of unneeded complexity in forming the contiguous wall structure. In addition, bonding a contiguous wall structure to a substrate results in a very rigid package, which can lead to package failure when thermally stressed due to differences in the coefficient of thermal expansion (CTE) between the heat spreader and substrate materials.
Second, the bond line thickness 104a between the integrated circuit 103a and the heat spreader 105a is strongly influenced by the size of the metal particles 206a that are a constituent of the thermal interface material used to form the bond between the integrated circuit 101a and the heat spreader 105a. This works well as long as the particle size 206a is relatively uniform. However, referring to FIG. 2b, when there is a wide distribution of metal particle 206b size, a condition called “die tilting” may occur. Die tilting is caused by a relatively large particle 206b in the thermal interface material becoming wedged in between the integrated circuit 101b and the heat spreader 105b. When a clamping pressure is applied to the heat spreader 105b during the manufacturing process, the large particle 206b causes an uneven distribution of force across the face of the integrated circuit 101b, either causing the device 103b to tilt to relieve the stress, or perhaps to crack. This die tilting or cracking may lead to either poor device reliability or to complete failure.
Therefore, what is needed is a heat spreader that will meet or exceed today's heat spreading performance requirements while being both easier to manufacture and designed to better control the bond thickness between the heat spreader and the device, leading to reduced cost and better integrated circuit reliability.